Method and apparatus for offswitching crossed field switch device

ABSTRACT

Crossed field switch device is offswitched by applying an offswitching magnetic field pulse to rapidly reduce the net magnetic field to below the critical value and to maintain the net magnetic field below the critical value for an extended period. This is accomplished by discharging parallel capacitors, the first being directly connected to the offswitching pulse coil, and the second having a time-delaying inductance connected serially therewith.

United States Patent 1 1 1111 3,749,978 Gallagher 5] July 31, 1973 METHOD AND APPARATUS FOR OFFSWITCHING CROSSED FIELD SWITCH Primary ExaminerRoy Lake DEVICE Assistant Examiner-James B. Mullins I75] Inventor: Hayden E. Gallagher, Attorney-W. H. MacAlhster, Jr. et al.

Malibu, Calif.

I73] Assignee: Hughes Aircraft Company, [57] ABSTRACT Culver W Callf- Crossed field switch device is offswitched by applying [22] Filed; May 26 1972 an offswitching magnetic field pulse to rapidly reduce the net magnetic field to below the critical value and to 1 l PP N05 2571106 maintain the net magnetic field below the critical value for an extended period. This is accomplished by dis- 521 U.S. c1. 315/236, 315/348 charging parallel capacitors, the first being directly 51 1111.01. H05b 37/00 connected to the Offswitching Pulse i nd h c- [58] Field of Search 315/236, 344, 345, 0nd having time-delaying inductance connected Seri- 315/346, 348; 313/154, 161; 328/65 y therewith- [56] References cued 7 Claims, 14 Drawing Figures UNITED STATES PATENTS 3,678,289 7/1972 Lutz et al 313/161 X Switching Signal Generator 20E, r--1 22 24 l O l g I A Lkilioi \76 [1 .Tao\ T78 T 64 1 PATENTEDJU s. 749.978

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METHOD AND APPARATUS FOR OFFSWITCHING CROSSED FIELD SWITCH DEVICE FIELD OF THE INVENTION This invention is directed to method and apparatus for the offswitching of crossed field switch devices.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic electric circuit which includes a crossed field'switch together with its main magnetic field supply and its switching magnetic field supply, in accordance with the prior art.

FIG. 2 is a graph of voltage versus magnetic field showing the conductivity conditions of a crossed field switch.

FIG. 3 is a graph of the main field strength of a crossed field tube versus time, in accordance with the prior art.

FIG. 4 is a graph of the switching magnetic field versus time in accordance with the prior art.

FIG. 5 is a graph of the net magnetic field versus time, in accordance with the prior art.

FIG. 6 is a graph of the voltage across the switching tube versus time, in accordance with the prior art.

FIG. 7 is a graph of the current through the crossed field tube versus time, in accordance with the prior art.

FIG. 8 is a schematic electric circuit incorporating a crossed field switch together with its main magnetic field and switching magnetic field supply, in accordance with this invention.

FIG. 9 is a graph of the current through the switching field magnetic coil versus time, in accordance with this invention.

FIG. 10 is a graph of the switching magnetic field strength versus time, in accordance with this invention.

FIG. 11 is a graph of the net magnetic field versus time, in accordance with this invention.

FIG. 12 is a graph of the voltage across the offswitching crossed field device versus time, in accordance with this invention.

FIG. 13 is a graph of the current through the crossed field switch versus time during offswitching in accor dance with this invention.

FIG. 14 is a schematic electric circuit showing a plurality of the crossed field switches in series which are to be offswitched in accordance with the method and apparatus of this invention.

THE PRIOR ART Crossed field switch devices are known in the prior art. They have been known in the past as laboratory devices and as small current devices. In the recent past, and in the present status otldevelopment, they are being improved as to various characteristics thereof. For example, G. A. G. Hofmann and R. C. Knechtli U.S. Pat. No. 3,558,960 is directed to increasing the charge that can be transmitted before internal conditions adversely change. G. vA. G. Hofmann U.S. Pat. No. 3,604,977 is directed to electrode configuration in such switches. M. A. Lutz and R. C. Knechtli U.S. Pat. No. 3,638,061 is another patent directed to improving the operating conditions of such crossed field switching devices The basic prior art switch devices are capable of offswitching direct current. Therefore, the crossed field switching device is capable of being incorporated within a circuit which is capable of acting as a circuit breaker. Such circuit breakers, in addition to simple offswitching, of which the crossed field switch is capable, are also directed to the employment of in-line switch devices for continuous current passage and means for absorbing energy upon circuit interruption. K. T. Lian U.S. Pat. No. 3,534,226 is one example of a circuit breaker into which a crossed field switch can be incorporated. K. T. Lian and W. F. Long U.S. Pat.

- No. 3,641,358 is another such patent. Additionally, W.

Knauer U.S. Pat. No. 3,657,607 is another patent which employs a highspeed DC switch of the crossed field switch type in a circuit breaker arrangement. The disclosures of these patents are incorporated herein by this reference.

In FIG. 1 a crossed field switch device is schematically indicated at 10. It is a device of the nature described in the patents listed above and is connected by lines 12 and 14 into a circuit which incorporates a source of power and a load, and may incorporate impedance increasing circuitry of the nature described in the patents listed above. Main field coil 16 is continuously supplied, while the crossed field switch device 10 is conducting, with current from power source 18, through switch 20 and serially connected resistor 22 and inductor 24. The resistor and inductor are included to limit current and current changes and may not be required under some circumstances. This circuitry and main field coil 16 comprises a main field supply which supplies a main magnetic field to crossed field switch device 10, as indicated by line 26 in FIG. 3. The main field strength is adequate to provide a magnetic field such that the interelectrode space of the crossed field switch device 10 is in conductive condition. While an electrically powered solenoid coil is indicated as being the source of the main magnetic field, it is clear that permanent magnet can be employed for this purpose.

FIG. 2 indicates the conductive region 28 for a particular crossed field switch device. It should be noted that the magnetic field strength B in FIG. 2 is within the conductive region 28 under some operating conditions, such as at point P and this corresponds in field strength to the main field strength indicated in FIG. 3.

As discussed in the patents indicated above, the manner of ofi'switching the crossed field switch is to reduce the magnetic field strength in the interelectrode space so that the conditions in the space are outside of the conductive region indicated in FIG. 2. One way of accomplishing this is to provide a pulsed offswitching magnetic field such as by the exemplary offswitching pulse circuitry generally indicated at 30 in FIG. 1. In this schematic circuitry, power supply 32 charges capacitor 34 through current limiting resistor 36. When the offswitching pulse is desired, switching signal generator 38 turns on the capacitor discharge switch 40, which maybe in the form of an SCR, to permit discharge of capacitor 34 through offswitching magnetic field coil 42. The parallel diode prevents reverse voltage buildup of field decay. This provides an offswitching magnetic pulse, indicated by the line 44 in FIG. 4, in the interelectrode space. The ofi'switching'pulse field is in the opposite magnetic direction to the main field so that the total or net magnetic field in the interelectrode space is indicated by line 46 in FIG. 5.

The net or total magnetic field in the interelectrode space decreases to a value below B,,, which is the toe of the conductive zone in FIG. 2. This means that the condition in the interelectrode space is such that conductivity ceases. As a result of a cessation in conductivity, the voltage across the switching device rises, as indicated by line 48 in FIG. 6. The plot of net magnetic versus voltage across the switch device during the offswitching pulse is indicated by line 50 in FIG. 2. The current through the device is cut by this offswitching, as indicated by line 52 in FIG. 7.

SUMMARY In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a method and apparatus for offswitching a crossed field device comprising maintaining the interelectrode space in the crossed field device in a nonconductive condition for an extended period of time so that the interelectrode conditions are maintained away from conductive conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENT It was discovered that upon offswitching the glow discharge of normal conductivity changed to an are discharge principally at the boundary between the conducting conditions and the non-conducting conditions ofa crossed field switch device. Some experimental results indicate that glow-to-arc transition switching values decreased with a faster decay of the magnetic field to the zero level. A possible explanation for this decreased amount of switching failure by glow-to-arc transition is that with a lower magnetic field in the interelectrode space, the charged particles are swept out of the interelectrode space at a lower interelectrode voltage. This lower interelectrode voltage and the resultant lower power density at the electrode surfaces appears to reduce the rate of switching failures. Switching failures, in addition to those caused by glow-to-arc transitions, can occur with the tubes continuing to conduct in the glow mode after the offswitching pulse period. The tendency of the tube to become conducting in the glow mode is related to the proximity of its instantaneous operating point to the boundary between the conducting and non-conducting regions indicated in FIG. 2. This was unexpected, because it was previously believed that the regions were well defined, both when defined in the manner of FIG. 2 and when defined as a Paschen curve.

FIG. 8 schematically illustrates the apparatus for offswitching the crossed field switch in accordance with this invention. The crossed field switch 10 with connecting lines 12 and 14 is the same as that illustrated in FIG. 1. Furthermore, main field coil 16 with its supply from power source 18 through switch 20, resistor 22 and inductor 24 is the same as that illustrated in FIG. 1. Furthermore, the main magnetic field supply can be by means of a permanent magnet. The result is a main field as illustrated in FIG. 3. The main field, previously described, is sufficient that when an electric field of sufficient strength is applied across the electrodes of the crossed field switch 10 the switch conducts, as previously described.

The offswitching magnetic field pulse is supplied by the offswitching circuitry 54, in accordance with this invention, as illustrated in FIG. 8. Again, an offswitching signal is produced by an offswitching signal generator 38. This may be connected to directly actuate switch 56, but in the preferred embodiment, to aid in permitting a plurality of the switch devices 10 to be serially connected, the switching signal generator 38 is electrically isolated from switch 56. This is accomplished by connecting the switching signal generator 38 to operate a light 58, for example, a light emitting diode. The light is connected through a dielectric light pipe 60 made of methyl mcthacrylate, for example, or similar material. The emitted light impinges upon light detector 62 which is suitably connected to control the gate of the SCR which is the preferred form of switch 56.

Power supply 64 is connected on the one side through line 66 to magnetic pulse switching field coil 68. The other side of the power supply is connected by line 70 through current limiting resistor 71, serially connected inductors 72 and 74, through SCR switch 56 to the other side of switching field coil 68. Switching field coil 68 is paralleled by diode 76 which prevents a buildup of back voltage due to decaying magnetic field. Capacitor 78 is connected between lines 66 and 75. A capacitor 80 is connected to line 66 and between inductors 72 and 74. Capacitor 82 is connected to line 66 and between inductor 74 and SCR switch 56.

Power supply 64 supplies capacitor charging current through the charging current limiting resistor 71, so that when switch 56 is off the capacitors 78, 80 and 82 are charged. Assuming initially that the crossed field tube 10 is in the conducting state with a main magnetic field strength of B,,, a signal from switching signal generator 38 turns on switch 56. Thereupon, capacitor 82 discharges through the switch and through switching field coil 68. The discharge of capacitor 82 is without delay except for the normal inductance in offswitching coil 68. This inductance can be made lower than microhenries by using as few as one to two turns. The discharge of capacitor 80 is time-delayed by the inductance 74 and serves to maintain the offswitching pulse fora longertime period. Similarly, the discharge of capacitor 78 is further delayed by the serial inductances 72 and 74 to continue the offswitching pulse for a longer time period.

In the prior art, the single capacitor 34 caused a low slope of the total magnetic field with respect to time, as illustrated in FIG. 5. With the low slope, it is clear that a small change in the main field strength 13,, causes a substantial change along the time scale. This changes the time at which the field is reduced to the critical value. The value t in FIGS. 5, 6 and 7 is the nominal offswitching time. Changes in time of offswitching due to the magnetic differences mentioned above, and due to possible physical differences in the tube are represented as At. Thus, offswitching time can vary a substantial amount. FIGS. 6 and 7 indicate the variations in voltage and current due to these offswitching time changes represented in the range At. The shape or pulse width of the switching field is such that the voltage across the tube must increase to a value within the non-conducting region during the time the total field is below 8,. This is represented in FIG. 5 as between the time t, and 1,. In FIG. 2, this is illustrated as moving from P, to P, The shape of the switching field is determined by the inductance of coil 42 and the capacitance 34 in the switching circuit of FIG. 1. The pulse width equals 1r V LC. The value of the inductance is determined by the main field B However, the capacitance C can be varied. If the capacitance C is increased so that the period t, I is increased, a low dV/dt across the switch tube is permissable, then the slope in FIG.

5 is low and hence the time jitter in offswitching increases. Low dV/dt is required by the transmission system (application for these switch tubes) and to increase the reliability of the switch tubes (offswitching is more reliable at lower dV/dt).

. This is overcome by the offswitching circuitry of FIG. 8. The inductance of offswitching coil 68 and the capacitance of first capacitor 82 determine the rate of rise of the switching pulse. The remaining network inductors 74 and 72 and capacitors 80 and 78 determine the pulse width. Thus, the rate of rise of the switching current pulse is proportional to L C and the pulse width is proportional to the sum of the inductors and the capacitors, V 2 (L,,) 2 (C,,). A switching current waveform obtained with a typical network is shown in FIG. 9, where the current to the offswitching pulse coil 68 is indicated at 84. A large initial current is provided by capacitor 82, and this initial current is required to induce the eddy currents in the cathode and anode cylinders during the field change. The resultant offswitching field pulse indicated by curve 86 in FIG. 10, and the total magnetic field shown by curve 88 in FIG. 11 does not show this overshoot. Thus, the initial capacitor is designed to provide enough energy to induce the eddy currents. The network of capacitors and inductors is tailored to supply this high initial current and the lower current which maintains the offswitching magnetic field pulse for the remainder of the pulse.

The rapid rate of rise of the switching pulse by the large amount of energy provided by capacitor 82 provides a steeper slope in the total field of curve 88 during offswitching. Thus, the magnetic switching pulse passes through the critical AB field value in much less time. Thus, At in FIG. 11 is much smaller than the At in FIG. 5. The advantage of this steep slope in the offswitching field is shown by curves 90 and 92 in FIGS. 12 and I3 where the small At provides for less of a tube voltage difference and less of a tube current difference. Thus, the crossed field switch tube can be offswitched with more precision with respect to time.

In addition to providing the steep offpulse form of FIG. 11, the pulse width is longer in time so that the magnetic field is maintained below the critical value B, for a longer time. This permits the voltage versus field strength curve of FIG. 2 to pass along the line 90 from P to P and stay away from the conductive region. It has been noted that the point P is close to the boundary between the regions and thus, sometimes there was a reinitiation of the glow discharge whereby offswitching could not be obtained.

FIG. M illustrates the crossed field switch device 10 connected in series with crossed field switch devices 92, 94 and 96. Each of these devices is identical and they are serially connected to provide for the ability to offswitch against higher line voltages. Terminals are provided to connect the series combination into a circuit. Serially connected capacitors 98, I00, 102 and I04 are each individually connected in parallel to the serially connected crossed field switches. These capacitors permit a certain amount of energy to be absorbed when one of the crossed field switches turns off sooner than one of the others in the series combination. The parallel capacitor limits the voltage rise dV/dt across the tube. If there was a large difference in offswitching time, the entire line voltage would be applied across the first switch to be offswitched, with a resultant overvoltage and failure. The more accurate control of the offswitching time in accordance with this invention reduces the required capacitor size and increases reliability.

Switching failures by having the glow discharge in the conducting switch device transition to an arc discharge occurs most often at the time of offswitching. Some experimental results indicate that this type of failure is reduced with a faster decay of the magnetic field to near the zero level. A possible explanation for this decreased tendency for arcing is that with the lower magnetic field, the charged particles are swept out of the interelectrode space at alower interelectrode voltage. The lower voltage and the resulting lower power density at the electrode surfaces would be expected to reduce the number of times that arcs would appear. Thus, offswitching reliability of the individual tube is also en hanced by having a steep front on the offswitching pulse.

As a specific example of a device, the crossed field switch 10 is a double cylindrical device with an interelectrode radial space of 2 cm. A typical inner electrode has a diameter of 50 cm and an area of 7,000 square cm. The electrodes can be made of tantalum or stainless steel. The interelectrode space is filled with helium gas at 0.05 Torr. The main magnetic field has a field strength B, of I00 Gauss. A device of this nature is capable of carrying 2,000 amperes and offswitching against 100 kv. v

The offswitching coil 68 has one turn and has an inductance of 10 henries. It is supplied with capacitors 78, 80 and 82 which have capacitance of 50 [LF each.

Inductances 72 and 74 have inductances of 3 microhenries each. Power supply 64 charges the capacitors to 6 kv. This network provides a maximum switching current of 30 k amps with a switching field of about Gauss and having a substantially flat top as illustrated in FIG. 10 for about 100 microseconds. The capacitors 98 through 104 limit the voltage rise across the crossed field switch devices during offswitching to about I kv per microsecond, so that the pulse width is sufficient to keep conditions out of the conductive region. By this means reliable offswitching is obtained.

This invention has been described with respect to its preferred embodiment. It is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

What is claimed is: 1. An apparatus for offswitching comprising: a crossed field switch device for connection into an electric circuit to offswitch the electric circuit;

magnetic means for applying a substantially constant magnetic field to the interelectrode space of said crossed field switch device at a magnetic field strength above the critical magnetic field value of the crossed field switch device, the improvement comprising:

means for applying a magnetic field offswitching pulse to the interelectrode space of the crossed field switch device of such polarity to decrease the net magnetic field below the critical magnetic field value for offswitching the crossed field switch device and for producing an offswitching pulse having a steeper offswitching wave front than that produced by discharge of a single capacitor through an offswitching magnetic pulse coil. 2. The apparatus of claim 1 wherein said means produces an offswitching pulse of such shape as to produce a net field in the interelectrode space below the critical conduction value for a longer length of time than the discharge of a single capacitor through an offswitching pulse coil.

3. The apparatus of claim 1 wherein there are a plurality of said crossed field switches connected in series in a circuit to be offswitched and there is means for applying a main magnetic field to each of said crossed field switch devices, and there is one of said means for applying an offswitching magnetic field pulse to each of said crossed field switch devices.

4. An apparatus for offswitching comprising: a crossed field switch device for connection into an electric circuit to offswitch the electric circuit;

magnetic means for applying a substantially constant magnetic field to the interelectrode space of said crossed field switch device at a magnetic field strength above the critical magnetic field value of the crossed field switch device, the improvement comprising:

means for applying a magnetic field offswitching pulse to the interelectrode space of the crossed field switch device of such polarity to decrease the net magnetic field below the critical magnetic field value for offswitching the crossed field switch device, said means for producing an offswitching pulse comprising an electromagnetic coil positioned with respect to the interelectrode space to produce an offswitching magnetic pulse therein, and a plurality of capacitors connected in parallel to each other and in series with said coil so that discharge of said capacitors through said coil causes an offswitching pulse.

5. The apparatus of claim 4 wherein one of said capacitors is directly connected to said offswitching pulse coil and another of said capacitors is connected through an inductor to said offswitching pulse coil.

6. The apparatus of claim 4 wherein there are a plurality of said crossed field switches connected in series in a circuit to be offswitched and there is means for applying a main magnetic field to each of said crossed field switch devices, and there is one of said means for applying an offswitching magnetic field pulse to each of said crossed field switch devices.

7. The method of offswitching a crossed field switch device which is connectable in a circuit for the offswitching of the circuit, said crossed field switch device having a main magnetic field means for producing a magnetic field in the interelectrode space above the critical value and having an offswitching magnetic field coil for producing an offswitching magnetic field'pulse of such polarity to reduce the net magnetic field in the interelectrode space below the critical value comprising the steps of:

producing an offswitching magnetic field pulse in the interelectrode space having a steep wave front to minimize the time period which the net magnetic field passes through the critical range by discharging a first capacitor through the magnetic field coil; and

maintaining the offswitching magnetic field pulse for a sufficient length of time by discharging a second capacitor through the magnetic field coil to maintain the tube in the non-conducting condition until the inter-electrode voltage rises well above conductive conditions.

* k i i i 

1. An apparatus for offswitching comprising: a crossed field switch device for connection into an electric circuit to offswitch the electric circuit; magnetic means for applying a substantially constant magnetic field to the interelectrode space of said crossed field switch device at a magnetic field strength above the critical magnetic field value of the crossed field switch device, the improvement comprising: means for applying a magnetic field offswitching pulse to the interelectrode space of the crossed field switch device of such polarity to decrease the net magnetic field below the critical magnetic field value for offswitching the crossed field switch device and for producing an offswitching pulse having a steeper offswitching wave front than that produced by discharge of a single capacitor through an offswitching magnetic pulse coil.
 2. The apparatus of claim 1 wherein said means produces an offswitching pulse of such shape as to produce a net field in the interelectrode space below the critical conduction value for a longer length of time than the discharge of a single capacitor through an offswitching pulse coil.
 3. The apparatus of claim 1 wherein there are a plurality of said crossed field switches connected in series in a circuit to be offswitched and there is means for applying a main magnetic field to each of said crossed field switch devices, and there is one of said means for applying an offswitching magnetic field pulse to each of said crossed field switch devices.
 4. An apparatus for offswitching comprising: a crossed field switch device for connection into an electric circuit to offswitch the electric circuit; magnetic means for applying a substantially constant magnetic field to the interelectrode space of said crossed field switch device at a magnetic field strength above the critical magnetic field value of the crossed field switch device, the improvement comprising: means for applying a magnetic field offswitching pulse to the interelectrode space of the crossed field switch device of such polarity to decrease the net magnetic field below the critical magnetic field value for offswitching the crossed field switch device, said means for producing an offswitching pulse comprising an electromagnetic coil positioned with respect to the interelectrode space to produce an offswitching magnetic pulse therein, and a plurality of capacitors connected in parallel to each other and in series with said coil so that discharge of said capacitors through said coil causes an offswitching pulse.
 5. The apparatus of claim 4 wherein one of said capacitors is directly connected to said offswitching pulse coil and another of said capacitors is connected through an inductor to said offswitching pulse coil.
 6. The apparatus of claim 4 wherein there are a plurality of said crossed field switches connected in series in a circuit to be offswiTched and there is means for applying a main magnetic field to each of said crossed field switch devices, and there is one of said means for applying an offswitching magnetic field pulse to each of said crossed field switch devices.
 7. The method of offswitching a crossed field switch device which is connectable in a circuit for the offswitching of the circuit, said crossed field switch device having a main magnetic field means for producing a magnetic field in the interelectrode space above the critical value and having an offswitching magnetic field coil for producing an offswitching magnetic field pulse of such polarity to reduce the net magnetic field in the interelectrode space below the critical value comprising the steps of: producing an offswitching magnetic field pulse in the interelectrode space having a steep wave front to minimize the time period which the net magnetic field passes through the critical range by discharging a first capacitor through the magnetic field coil; and maintaining the offswitching magnetic field pulse for a sufficient length of time by discharging a second capacitor through the magnetic field coil to maintain the tube in the non-conducting condition until the inter-electrode voltage rises well above conductive conditions. 